This application is based on applications Nos. 2001-053288, 2001-097713, 2001-097714, and 2001-100387 filed in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a photoelectric conversion device having a substrate with numerous crystalline semiconductor particles deposited thereon, and a method of manufacturing the device. This photoelectric conversion device is used for solar cells and the like.
2. Description of the Related Art
Photoelectric conversion devices using such a raw material as silicon have lower potential to pollute the environment in their production and operation processes when compared with other power generation means. In addition, they are economical since their main energy source is the sun. A photoelectric conversion device with relatively high efficiency that has been in wide practical use is manufactured using wafers as the starting material, in which relatively cheap metal-grade silicon is subjected to various high-purification processes and finally formed into monocrystal or polycrystalline silicon by the Czochralski method or the like, which is then sliced into individual wafers. This conventional-type photoelectric conversion device can receive light at the whole surface of its conversion element and therefore has high conversion efficiency. However, due to the costly starting material, the wafer itself, the solar cell device is bound to be expensive hindering its diffusion.
With the background as above, another photoelectric conversion device which has a substrate with numerous spherical crystalline semiconductor particles of one conductivity type mounted thereon has been devised as a lower-cost photoelectric conversion device (ball solar) in which the amount of semiconductor material usage has been reduced.
The structure of a photoelectric conversion device in the prior art is shown in FIG. 20. This photoelectric conversion device comprises a substrate 91 serving as lower electrode, numerous crystalline semiconductor particles 95 of one conductivity type deposited thereon, an insulator layer 92 interposed among the crystalline semiconductor particles 95, and a semiconductor region 93 of the opposite conductivity type and a protective layer 94 which are laminated together over the crystalline semiconductor particles 95.
In this structure, the use of the spherical crystalline semiconductor particles 95 allows for small usage of semiconductor and cost reduction. However, light incident on the areas among the crystalline semiconductor particles 95 cannot be converted into electricity, which causes the problem of insufficient conversion efficiency.
Now, this is more specifically discussed, picturing the daily movement of the sun. For example, when silicon spheres are most densely and horizontally disposed, sunlight does not directly strike the areas among the closely laid silicon spheres until the sun rises up to around 30 degrees from the horizontal. Accordingly, the utilization ratio of sunlight is relatively high during this period. However, as the sun rises, the ratio of the sunlight that is incident on the areas among the closely laid silicon spheres increases. Consequently, the closer to the right angle the incident angle of the sunlight is, the lower the ratio of utilized sunlight energy to the whole incident sunlight energy becomes in the above mentioned photoelectric conversion device. In other words, since it fails to sufficiently utilize the sunlight that has reached the areas among the silicon spheres, it cannot exhibit high conversion efficiency.
Also, in the structure of the photoelectric conversion device shown in FIG. 20, the crystalline semiconductor particles 95 have smooth surfaces so that light incident on the surfaces of the particles is partly reflected and lost causing the conversion efficiency to drop.
Meanwhile, a generally used insulator layer 92 is an oxide layer which is formed by thermal oxidation, Vapor-phase Growth or the like. When it is formed by thermal oxidation, the processing temperature needs to be high enough in order to obtain an insulator layer 92 with sufficient insulation and thickness, and the processing time is long accordingly. Therefore, under some conditions for forming the oxide layer, the temperature of the metal surface rises so high that the substrate and silicon spheres melt. Similar problems occur when it is formed by vapor phase techniques.
In the structure of the photoelectric conversion device illustrated in FIG. 20, the substrate 91 that serves as lower electrode and the crystalline semiconductor particles 95 are in contact with each other at acute angles xcex8 (less than 90 deg.). For this reason, when the insulator layer 92 is formed between the substrate 91 serving as lower electrode and the crystalline semiconductor particles 95 using anodic oxidation method, the insulator layer 92 is not evenly distributed and voids are generated. It is therefore difficult to form a reliable insulator layer, which causes current leak to occur and deteriorates the function as a photoelectric conversion device.
It is an object of this invention to provide a photoelectric conversion device with high conversion efficiency capable of sufficiently utilizing energy of the sunlight incident on the device.
Another object of this invention is to provide a photoelectric conversion device having a reliable insulator layer which does not create voids in the joining area between the substrate and crystalline semiconductor particles.
Still another object of this invention is to provide a method of manufacturing a photoelectric conversion device that allows an insulator layer to be formed at relatively low temperatures without using thermal oxidation or Vapor-phase Growth.
A. A photoelectric conversion device according to the present invention comprises: a lower electrode; numerous crystalline semiconductor particles of one conductivity type deposited on the lower electrode; an insulator interposed among the crystalline semiconductor particles; and a semiconductor layer of the opposite conductivity type provided over the crystalline semiconductor particles, wherein a pyramidal projection is provided between the crystalline semiconductor particles and a lateral face of the pyramidal projection faces one of the crystalline semiconductor particles.
In this photoelectric conversion device, since a pyramidal projection with a lateral face that faces a crystalline semiconductor particle is provided between the crystalline semiconductor particles, light incident on the area between the crystalline semiconductor particles is reflected or refracted by the pyramidal projection and enters the crystalline semiconductor particles. Accordingly, this invention can provide a photoelectric conversion device with high photoelectric conversion efficiency.
B. Another photoelectric conversion device according to this invention comprises: a lower electrode; numerous crystalline semiconductor particles of one conductivity type deposited on the lower electrode; an insulator interposed among the crystalline semiconductor particles; and a semiconductor layer of the opposite conductivity type provided over the crystalline semiconductor particles, wherein the light-receiving surface of the insulator includes a recess formed at a position between the crystalline semiconductor particles.
In this photoelectric conversion device, since a recess is formed on the light-receiving surface of the insulator interposed between the crystalline semiconductor particles, light rays that have stricken the surface of the insulator and have been reflected from there are bent toward the crystalline semiconductor particles by the recess and absorbed by the crystalline semiconductor particles. Accordingly, the utilization rate of the sunlight that has arrived among the crystalline semiconductor particles is improved, and in particular, the sunlight is effectively utilized when it is vertically incident. A photoelectric conversion device with high conversion efficiency can be thus realized.
C. Another photoelectric conversion device according to this invention comprises: a lower electrode; numerous crystalline semiconductor particles of one conductivity type deposited on the lower electrode; an insulator interposed among the crystalline semiconductor particles; and a semiconductor layer of the opposite conductivity type provided over the crystalline semiconductor particles, wherein the crystalline semiconductor particles comprise a roughed surface.
The roughed surface of the crystalline semiconductor particle makes it easier for light incident on the crystalline semiconductor particle to enter the PN-junction within the crystalline semiconductor particle. Also, light reflected from the roughed surface is scattered in various directions so that part of the light is directed to adjacent crystalline semiconductor particles. Accordingly, the photoelectric conversion efficiency of the photoelectric conversion device is improved. In addition, the adhesion between the crystalline semiconductor particles and the substrate is enhanced.
D. Another photoelectric conversion device according to this invention comprises: a lower electrode; numerous hemispherical crystalline semiconductor particles of one conductivity type deposited on the lower electrode; an insulator interposed among the hemispherical crystalline semiconductor particles; and a semiconductor layer of the opposite conductivity type provided over the hemispherical crystalline semiconductor particles, wherein the surfaces of the hemispherical crystalline semiconductor particles and the lower electrode make an angle of 90 degrees or more.
A method of manufacturing a photoelectric conversion device according to this invention comprises the steps of: joining numerous hemispherical crystalline semiconductor particles of one conductivity type to a substrate having a conductive region at the surface; forming an insulating region among the hemispherical crystalline semiconductor particles by subjecting the conductive region at the surface of the substrate to anodic oxidation; and forming a semiconductor layer of the opposite conductivity type over the crystalline semiconductor particles and the insulating region.
Since the above mentioned photoelectric conversion device comprises crystalline semiconductor particles whose surfaces make an angle of 90 degrees or more with the substrate (lower electrode), it allows formation of a good insulator layer that holds insulation at the junction area between the substrate and the crystalline semiconductor particles when the surface of the substrate is oxidized by anodic oxidation or the like for forming an insulator. Also, the adhesion between the insulator layer and the substrate is enhanced in this arrangement, making it possible to provide a photoelectric conversion device with high conversion efficiency.
According to the above-mentioned method of manufacturing a photoelectric conversion device, an insulator layer is formed by subjecting the conductive region at the surface of the substrate to anodic oxidation. Therefore, it is possible to form a good insulator layer that holds insulation at the junction area between the substrate and the crystalline semiconductor particles at relatively low temperatures. The adhesion between the insulator layer and the substrate is also enhanced. It is therefore possible to provide a photoelectric conversion device capable of restraining leakage current and achieving high conversion efficiency.
Now, structural details of the present invention are described with reference to the drawings.